There are a couple of possibilities:
1 – your site hasn’t fully propagated yet
When you said you “created an installation of WordPress” it sounds like you have a completely new install with new username and password. If you are using that new login and you can’t get it, DNS probably has not completely propagated. It can take a couple of days. You will know this is the case if you use your old login credentials and get in – that means DNS hasn’t fully switched over yet.
2 – you’re using the wrong login credentials
You can double-check your login credentials using phpMyAdmin. Go to the wp_users table to find your username. You can either reset the password from the login page, or directly here, by typing in a password and then making sure to use MD5 for that data so it encrypts it for you. Then once again you will go to the login screen and if it doesn’t work, you’ll know it’s DNS.
3 – wp-config settings are wrong
Last case, if your wp-config file is pointing to the wrong database – such as the old one – that will prevent you from getting in as well. Double-check that the file points to the correct db.